Electrosurgical snare

ABSTRACT

An electrosurgical snare, e.g. suitably sized for insertion down the instrument channel of an endoscope, arranged to radiate microwave frequency energy (e.g. having a frequency greater than 1 GHz) from an elongate conductive element within an area encircled by a retractable loop. The elongate conductive element and retractable loop may be independently slidable relative to a snare base at a distal end of a sleeve to provide an appropriate device configuration. By controlling the shape of the emitted microwave field, the risk of collateral thermal damage can be reduced.

FIELD OF THE INVENTION

The invention relates to a surgical snare, e.g. for use in a polypectomyprocedure. In particular, the invention may be concerned with medicalsnares suitable for insertion down the instrument channel of anendoscope (or any other type of scoping device used in thegastrointestinal (GI) tract or elsewhere in the human or animal body),and which include a means for introducing electromagnetic energy intobiological tissue.

BACKGROUND TO THE INVENTION

Polyps in the GI tract can be removed using a medical snare in anendoscopic procedure, e.g. using a colonoscope. In the case ofpedunculated polyps, the snare is passed over the polyp and tightenedaround the polyp's neck, which is then cut and the polyp removed. Thecutting process may be performed or enhanced by passing a radiofrequency(RF) current through the biological tissue. The current may alsofacilitate cauterization.

Sessile polyps can be removed in a similar manner. It is preferable to“plump up” such polyps before removal by injecting saline or sodiumhyaluronate, under the polyp to raise it away from the surrounding colonwall. This may help to reduce the risk of bowel perforation.

It is known to incorporate electrodes into the loop of a snare in orderto provide an integrated means for delivering the RF current. Bothmonopolar, for use with a separate ground pad attached to the patient,and bipolar arrangements are known.

A disadvantage of known RF cutting snares is the high level ofelectrical power (in particular the use of high voltages) needed toinitiate cutting action, as it carries with it the risk of unwantedthermal damage to the bowel wall. For example, the peak voltageassociated with monopolar and bipolar coagulation may be in excess of4,500 V and 450 V respectively.

SUMMARY OF THE INVENTION

At its most general, the present invention proposes snare structuresarranged to radiate microwave frequency energy (e.g. electromagneticenergy with a frequency of at least three orders of magnitude higherthan typical RF energy) within the area encircled by the snare's loop.By controlling the shape of the emitted microwave field, the risk ofcollateral thermal damage can be reduced. For example, typical peakvoltages in embodiments of the invention are 10 V or less. Moreover, theemitted microwave field can be more effective than an RF field atcoagulating blood.

According to a first aspect of the invention there is provided asurgical snare comprising: a retractable loop of conductive material forencircling an area containing biological tissue; a radiating structurearranged to radiate microwave frequency energy into the area encircledby the retractable loop; and a coaxial cable for conveying microwavefrequency energy to the radiating structure, the coaxial cablecomprising an inner conductor, an outer conductor surrounding andcoaxial with the inner conductor, and a dielectric material separatingthe inner conductor and the outer conductor, wherein the radiatingstructure comprises: an elongate conductive member connected to theinner conductor of the coaxial cable and being electrically insulatedfrom the outer conductor of the coaxial cable, and a snare base at adistal end for the coaxial cable, the snare base having a feed channelfor conveying a length of the conductive material that forms theretractable loop, wherein the elongate conductive member comprises adistal portion that protrudes into the area encircled by the retractableloop to act as a radiating microwave monopole antenna, and a proximalportion that extends through the snare base alongside the feed channel.With this radiating structure, microwave power may be launched into thearea encircled by the retractable loop both via the radiating microwavemonopole antenna and also via a travelling wave set up on the conductivematerial by coupling energy from the proximal portion of the elongateconductive element into the length of conductive material in feedchannel. Thus, the radiated microwave field may be directed intobiological tissue held by the snare. The snare base may comprise a pairof feed channels, each feed channel receiving a length of the conductivematerial that forms the retractable loop, e.g. on opposite side of theelongate conductive member.

The electrical length of the elongate conductive member may be about

$\frac{( {{2n} - 1} )\lambda_{L}}{4},$

where λ_(L) is the wavelength of the microwave frequency energy alongthe proximal portion of the elongate conductive element, i.e. thewavelength in the snare base, and n is a positive integer.

The snare base may comprise a plastic or dielectric casing that can beshaped to prevent the biological tissue held by the snare from beingforced down over the elongate conductive element. The snare base mayinclude other dielectric components, e.g. for controlling the positionof the snare or for permitting limited penetration of the elongateconductive element into the biological tissue.

Herein, “microwave frequency” may be used broadly to indicate afrequency range of 400 MHz to 100 GHz, but preferably the range 1 GHz to60 GHz, more preferably 2.45 GHz to 30 GHz or 5 GHz to 30 GHz. Specificfrequencies that have been considered are: 915 MHz, 2.45 GHz, 3.3 GHz,5.8 GHz, 10 GHz, 14.5 GHz and 24 GHz.

The surgical snare of the invention may be configured for insertion downan instrument channel of an endoscope, or may be arranged for use inlaparoscopic surgery or in a NOTES procedure.

One advantage of using microwave frequency energy is that the depth ofpenetration of the electric field into biological tissue is small, e.g.of the order of millimeters at the frequency of choice. The focused heatprofile depends on the square of the electric field, and theconductivity, density and specific heat capacity of the target tissuebeing treated. The microwave field emitted by the radiating microwavemonopole antenna is therefore naturally confined to biological tissue inthe region of the snare, thereby reducing the risk of collateral thermaldamage.

The microwave frequency energy in the invention may be for the purposeof coagulating blood, i.e. sealing blood vessels, in the tissueencircled by the snare, to assist in the overall removal process. Thestem of the polyp may be cut by the action of the retractable loop, i.e.the conductive material of the loop may comprise a sharp wire or thelike. Alternatively or additionally, a blade or other cutting structuremay be formed on the snare base, whereby drawing the biological tissuetowards the snare base by closing the loop causes cutting.

In another embodiment, the snare may be configured to receive both RFand microwave frequency energy. The snare may operate as a conventionalbipolar RF device to cut through the stalk, but with the added abilityto switch in the microwave frequency energy when coagulation isrequired. In this configuration, the conductive material of theretractable loop may comprise two conductors separated by an insulatorto deliver a local RF field. One or both of the same conductors may beused to deliver the microwave energy. The spacing between the twoconductors is preferably 0.5 mm or less and the diameter or width of theconductors is preferably 1.5 mm or less to form a practically usefuldevice. The conductors may be arranged in a co-planar configuration,where the active and the return are on the same surface or both surfacesand/or a configuration where electrodes are deposited onto a dielectricmay be employed where the active and return conductors alternate alongthe length of the loop.

The retractable loop may not be conductive along its whole length. Itmay be desirable to use a non-metallic, e.g. nylon, loop to snare andassist with cutting through the stalk. The radiating structure may beconfigured to operate only in a region at the snare base, i.e. at the“neck” of the snare. In this configuration, microwave energy may alsoassist with the mechanical cut. In one arrangement, the radiating neckmay take the form of a ‘V’ and in another, the radiating section (e.g.comprising a conductive section or coating on the loop) may form a partof the circumference of the loop, i.e. 45°, 90° or 180°.

Preferably, the length of the elongate conductive member and inparticular the length of proximal portion thereof is determined bymodeling because of the complicated and non-uniform structuresurrounding it at the neck of the snare, based on the microwavefrequency to be used.

If the snare is non-conductive, the elongate conductive member may beshaped to penetrate biological tissue. It may have a pointed distal end.It may be coated in a biocompatible material, e.g. PTFE or similar.Thus, the microwave frequency energy may be primarily radiated intoblood. The dielectric properties of blood may thus be used to determinethe properties of the radiating structure. For example, the relativepermittivity (dielectric constant) ε_(r) of blood at a frequency of 5.8GHz is 52.539. The loaded wavelength λ_(L) in this case may thus be 7.25mm. In general, the formula for calculating λ_(L) in a medium whoserelative permittivity is ε_(r) is

${\lambda_{L} = \frac{c}{f\sqrt{ɛ_{r}}}},$

where c is the speed of light and f is the microwave frequency. Thelength of the elongate conductive member may in this be close to any oneof 1.81 mm, 5.44 mm, or 9.06 mm, which are all odd multiples of aquarter loaded wavelength. Ideally the length may be greater than 1 mmand less than 6 mm. This range of lengths is commensurate with the sizeof the polyp structure that may be encountered in a polypectomy.

The coaxial cable may be encased in a sleeve suitable for insertionthrough the instrument channel of an endoscope. The coaxial cable mayextend between a proximal end, e.g. having a microwave connector forconnecting to a suitable microwave signal generator, and a distal end atwhich the radiating structure is located. The length of the coaxialcable may be suitable for endoscopic procedures, e.g. 2 m or more. Thesnare base may include an insulating cap at its distal end to ensureisolation between the outer conductor of the coaxial cable and theelongate conductive member.

The snare base may be attached at the end of the sleeve, whereby thecoaxial cable and the length of conductive material that forms theretractable loop may be movable (e.g. slidable) relative to the snarebase. The snare base may thus comprises a cylindrical plug element atthe distal end of the sleeve, wherein the plug element has a firstpassageway therethrough which provides the feed channel and a secondpassageway therethrough for conveying the coaxial cable.

The length of conductive material that forms the retractable loop mayhave a first end that is attached to a movement mechanism (e.g. push rodas explained below) for extended and retracting the retractable loop,and a second end that is attached (i.e. fixed) to the snare base.

In one embodiment, the snare base comprises a terminal cap fixed to thedistal end of the sleeve. The second end of the length of conductivematerial that forms the retractable loop may be attached to an outersurface of the coaxial cable. If the coaxial cable is slidable withinthe sleeve, this arrangement means that the second end of theretractable loop can be moved within the sleeve. This may facilitatefull retraction of the snare.

The retractable loop may be made from any suitable wire-like material,e.g. nitinol, nylon, metal wire or the like. Preferably the material hasshape retaining properties so that it automatically adopts a loopconfiguration upon being released from a retracted configuration. Thus,the retractable loop may comprise a wire that extends beyond the distalend of the coaxial cable, the wire being arranged to naturally adopt alooped shape between two ends located at the distal end of the coaxialcable. The retractable loop may be adjustable to vary the length of wirebetween the two ends. The looped shape may be free from irregularities.In particular, the looped shape may not require a distal hump or nibsuch as those commonly found on conventional surgical snares to ensurethat they retract in a predetermined manner. The invention may obviatethe need for such a nib through the use of nitinol as the loop material,and/or through the use of the deployment mechanisms set forth herein.Omitting the distal nib may ensure that the snare provides a cleanermechanical cut, which in turn provides a better en-bloc specimen forhistological assessment and facilitates complete excision of tissuecircled by the snare.

The retractable loop may be movable relative to the snare base, e.g.into and out of a storage channel formed in the sleeve surrounding thecoaxial cable. Preferably the retractable loop is be movable relative tothe coaxial cable. However, it may be possible for the retractable loopto be fixed relative to the coaxial cable and for retraction to beperformed by moving a tubular cover relative to the coaxial cable overthe loop.

A pull wire (or push rod) may be connected or formed integrally with theretractable loop. The pull wire may extend to the proximal end of thecoaxial cable to enable the operator to deploy the snare. The pull wiremay be connected to a slider mechanism (e.g. a manual slider mechanism)at the proximal end of the device. The pull wire may be conveyed fromthe proximal end to the distal end through a passageway in the sleeve.It is desirable for the translation between the length of movement ofthe slider at the proximal end and the opening and closing of the loop(or changes in diameter once it comes out of the end of the catheter ortube) to be consistent. A thin lubricious tube may be attached (e.g.glued) to the outer jacket of the coaxial cable to act as a guide forthe pull wire (or pull wires). Alternatively, a very thin walledheat-shrinkable tube could be used to attach the guide tube to thecoaxial cable. The guide tube preferably runs straight along the axis ofthe coaxial cable.

A multi-lumen tube may be inserted inside the structure to provideseparate channels or spaces for the pull wire (or pull wires) and thecoaxial cable. Alternatively, a single tube may be attached to the outerconductor of the coaxial cable to contain the pull wire to prevent thepull wire from becoming twisted around the coaxial cable.

The orientation of the loop may be related to the orientation of thepassageway in the sleeve. Thus, the plane of the loop may be adjustableby rotating the sleeve. Preferably the sleeve is a braided cable capableof transferring torque. A hand grip for rotating the sleeve may bemounted on, e.g. clamped to, it at the proximal end.

Alternatively, the pull wire may also used to activate a screw mechanismthat causes a linear to rotational translation, e.g. a lead screwarrangement, that can be used to rotate the loop. The same pull wire isused to open and close the loop or to push a loop made from a sprungmaterial out of or into a catheter or tube to allow the loop to open andclose.

The elongate conductive member may be retractable independently of theretractable loop. For example, the sleeve and the coaxial cable may bemovable relative to other to move the elongate conductive member betweena storage position in which it is surrounded by the elongate conductivemember and a use position it which it protrudes from the sleeve. Theretractable loop may be operable as a “cold” snare, i.e. a snare thatoperates without an accompanying microwave radiation field, when theelongate conductive member is in the storage position. In thisarrangement, the snare may be used as a mechanically tissue capture andcutting tool. Tissue, e.g. a polyp stalk, may be encircled by theretractable loop when in an extended configuration. Upon retracting theloop, the encircled tissue may be forced against a distal surface of thesnare base, whereupon the loop passes through the tissue to physicallycut it. The distal surface of the snare base thus provides a reactionsurface for the mechanical cutting action of the loop. The surface ofthe loop (or perhaps only the inner surface of the loop) may beroughened or sharpened to make this physical cutting action moreeffective. In some circumstances, use of the device as a “cold” snaremay be preferable, as it may reduce the risk of delayed bleeding.

The first aspect of the invention may also be expressed as anelectrosurgical apparatus comprising a microwave signal generator foroutputting microwave frequency energy, and a surgical snare as describedabove connected to receive the microwave frequency energy and deliver itthrough the coaxial cable to be emitting as a microwave frequency fieldby the elongate conductive member.

According to a second aspect of the invention, there is provided asurgical snare comprising: a retractable loop for encircling an areacontaining biological tissue; a radiating structure arranged to radiatemicrowave frequency energy into the area encircled by the retractableloop; and a coaxial cable for conveying microwave frequency energy tothe radiating structure, the coaxial cable comprising an innerconductor, an outer conductor surrounding and coaxial with the innerconductor, and a dielectric material separating the inner conductor andthe outer conductor, wherein the radiating structure comprises a curvedconductive portion partially bounding the area encircled by theretractable loop, the curved conductive portion being connected to theinner conductor of the coaxial cable and electrically insulated from theouter conductor of the coaxial cable to act as a radiating microwavemonopole antenna. The second aspect differs from the first aspect in thenature of the radiating structure, which in this case is a curvedconductive portion of the snare's loop, rather than an elongate elementprotruding into the loop. However, the curved conductive portion stillacts as a radiating microwave antenna to deliver the microwave frequencyenergy into tissue held within the loop.

The curved conductive portion may extend between two ends, which may bespaced at equal distances from the point at which the inner conductor ofthe coaxial cable is connected to the curved conductive portion. Thecurved conductive portion may thus be symmetrically arranged at thedistal end of the coaxial cable. Preferably, the electrical lengthbetween the ends of the curved conductive portion is

$\frac{( {{2n} - 1} )\lambda_{L}}{4},$

where λ_(L) is the wavelength of the microwave frequency energy whenpropagating through the biological tissue, and n is a positive integer.Thus, the length of the curved conductive portion can be determined inthe same way as the elongate conductive element of the first aspect.However, the structure of the second aspect not invasive. The length ofthe curved conductive portion may thus be longer that the elongateconductive element of the first aspect, e.g. 10 mm or more.

The curved conductive portion may comprise a pair of flexible prongsextending from the distal end of the coaxial cable. Each prong may be awire or tube having a thickness or diameter selected such that itexhibits some intrinsic elasticity. The prongs may be symmetricallymounted with respect to the point of connection with the inner conductorof the coaxial cable (i.e. the feed point). The prongs may thus act tosplit the microwave frequency power received from the coaxial cable. Theimpedance of the prongs may be selected so that, when connected inparallel with the coaxial cable, they match the impedance of the coaxialcable, i.e. if the impedance of the lines that formed the prongs was 50Ωand the length of the prongs were made to be an odd multiple of aquarter wavelength at the frequency of choice, then each prong will betransformed to an impedance of 100Ω at the feed point to provide anoverall parallel impedance of 50Ω, e.g. the same impedance as thecharacteristic impedance of the non-resonant co-axial line, to createthe matched condition. The same principle can be applied for differentload impedances.

The curved conductive portion may be movable relative to a tubular endcap between a stored configuration in which it is enclosed by the endcap and a deployed configuration in which it protrudes beyond a distalend of the end cap. The curved conductive portion may deform to fitinside the end cap. For example, the pair of prongs mentioned above maybend in towards each other. The outer diameter of the end cap may beless than 2.6 mm so that it can fit down the instrument channel of anendoscope. Thus, in the stored configuration, the curved conductiveportion may be deformed to have a width of less than 2.5 mm.

The end cap may be slidable relative to the coaxial cable, e.g. via apull wire that extends to the proximal end of the coaxial cable. Asabove, the coaxial cable may be encased in a sleeve, which may have apassageway formed therein for the pull wire. A multi-lumen tube may beinserted inside the main catheter or tube or sleeve.

The curved conductive portion may also act as a guide for theretractable loop. For example, the curved conductive element maycomprise a hollow tubular section with an opening at one end thereof.The retractable loop, which is preferably formed from a non-conductivematerial such as nylon in this embodiment, may extend through thetubular section and through the opening. Having a hollow section of thecurved conductive portion does not affect the propagation of themicrowave frequency energy because the skin depth at such frequencies issmall enough to require only a thin layer of conductive material, i.e.at 5.8 GHz, the skin depth or the depth into the material at which theelectric field has reduced to 37% of its peak value, is in the order ofmicrometers (1×10⁻⁶ m) for good conductors, e.g. silver or gold.

The curved conductive portion may have a hollow tubular section throughwhich the retractable loop extends at both ends thereof. However, in oneembodiment, the retractable loop has a first end that is fixed (e.g. bylaser welding) to one end of the curved conductive portion. The loopthen passes into the opening of a hollow tubular portion on the otherend of the curved conductive portion. The length of loop that protrudesfrom the opening may be adjustable, e.g. to adjust the area encircled bythe snare. The snare may be asymmetric in this embodiment. Adjustment ofthe retractable loop may be via a pull wire that passes back to theproximal end of the coaxial cable. The curved conductive portion mayhave a second opening at a side facing away from the area enclosed bythe loop.

Features of the first aspect expressed above may also be incorporatedinto the second aspect.

Similarly to the first aspect, the second aspect of the invention mayalso be expressed as an electrosurgical apparatus comprising a microwavesignal generator for outputting microwave frequency energy, and asurgical snare as described above in relation to the second aspectconnected to receive the microwave frequency energy and deliver itthrough the coaxial cable to be emitting as a microwave frequency fieldby the curved conductive portion.

According to a third aspect of the invention, there is provided asurgical snare comprising: a retractable loop for encircling an areacontaining biological tissue; a radiating structure arranged to radiatemicrowave frequency energy into the area encircled by the retractableloop; and a coaxial cable for conveying microwave frequency energy tothe radiating structure, the coaxial cable comprising an innerconductor, an outer conductor surrounding and coaxial with the innerconductor, and a dielectric material separating the inner conductor andthe outer conductor, wherein the radiating structure comprises aconductive portion formed in or on the retractable loop, the conductiveportion being connected to receive microwave power from the coaxialcable and configured to radiate the received microwave frequency energyinto the area encircled by the retractable loop.

The third aspect differs from the first and second aspect in that theradiating structure is actually part of the retractable loop itself. Forexample, part of the loop may be metallized, i.e. coating in aconductive material, and electrically connected to the inner conductorof the coaxial cable but electrically insulated from the outer conductorof the coaxial cable. The coaxial cable may include an insulating cap atits distal end. The inner conductor may protrude through the cap, butthe outer conductor may be insulated by the cap from everything on thedistal side of the cap. The protruding part of the inner conductor maybe electrically connected to the conductive portion by crimping or thelike.

To radiate efficiently into biological tissue, the electrical length ofthe conductive portion around the retractable loop may be

$\frac{( {{2n} - 1} )\lambda_{L}}{4},$

where λ_(L) is the wavelength of the microwave frequency energy whenpropagating through the biological tissue, and n is a positive integer.The length of the conductive portion may be determined using thetechnique described above with reference to the first aspect of theinvention.

In this aspect, the retractable loop may have a fixed end e.g. at thedistal end of the coaxial cable, and an adjustable end, e.g. connectedto a pull wire that may extend to a proximal end of the coaxial cable,where it is operable using a slider or the like. Similarly to the firstand second aspects above, the snare may include a slidable end cap,although this is optional in this case because only the retractable loopmay extend beyond the distal end of the coaxial cable.

The coaxial cable or sleeve may provide a torque-stable cable capable oftransferring a rotational movement effected at the proximal end of thedevice to the retractable loop. Rotation of the loop enables the snareto be easily position over and around a polyp. In this aspect, the outerjacket of the co-axial cable may be semi-rigid or a tube (catheter) maybe inserted over the outer jacket and form a tight fit.

Alternatively, rotation of the retractable loop may be achieved using amechanism located at the distal end of the cable that transforms alinear movement of the pull wire or the cable to a rotational movementof the loop to control the angle of the loop with respect to the stalkor stem of the polyp to enable the loop to be in the correct orientationto allow the loop to go around the polyp stalk. This linear torotational translation may also be used to control the opening andclosing (diameter) of the loop or the amount of loop that protrudes fromthe snare base.

In one embodiment, the conductive portion may be a “leaky feeder”, i.e.a length of coaxial cable that is shorted at its distal end and alongwhich portions of the outer conductor are periodically removed to permitradiation therefrom. The portions of removed outer conductor may beseparated by a distance of (2n−1)λ_(L)/4 where λ_(L) is the wavelengthof the microwave frequency energy when propagating through thebiological tissue, and n is a positive integer.

Whilst the conductive portion may be part of the retractable loopitself, preferably the retractable loop comprises a wire made frominsulating material that extends beyond the distal end of the coaxialcable, the wire being arranged to naturally adopt a looped shape betweentwo ends located at the distal end of the coaxial cable. The conductiveportion, which is connected to the inner conductor of the coaxial cable,may then be mounted on, e.g. bonded to or entwined with, the wire as itextends between the ends.

Features of the first aspect and second aspect expressed above may alsobe incorporated into the third aspect.

Similarly to the first and second aspects, the third aspect of theinvention may also be expressed as an electrosurgical apparatuscomprising a microwave signal generator for outputting microwavefrequency energy, and a surgical snare as described above in relation tothe third aspect connected to receive the microwave frequency energy anddeliver it through the coaxial cable to be emitting as a microwavefrequency field by the conductive portion.

In any of the aspects described above, the snare may include a fluiddelivery channel for introducing fluid such as saline on or close to thetreatment site, e.g. to assist with coagulation or to flush the area.The fluid delivery channel may be provided in the sleeve encasing thecoaxial cable, or it may be provided in the inner conductor of thecoaxial cable, e.g. by making it hollow.

The pull wire for the retractable loop is preferably may from aninsulating material to avoid capacitive coupling with the coaxial cableas it extends through the passage in the sleeve.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in detail below withreference to the accompanying drawings, in which:

FIGS. 1A and 1B show a schematic cross-sectional view of a surgicalsnare that is a first embodiment of the invention, in a deployed andretracted position respectively;

FIGS. 2A and 2B show a schematic cross-sectional view of a surgicalsnare that is a second embodiment of the invention, in a deployed andretracted position respectively;

FIGS. 3A and 3B show a schematic cross-sectional view of a surgicalsnare that is a third embodiment of the invention, in a deployed andretracted position respectively;

FIG. 4 is a perspective view of a model surgical snare used to simulatethe microwave delivery performance of the invention;

FIG. 5 is a graph showing return loss (impedance match) into blood forthe model surgical snare shown in FIG. 4;

FIG. 6 is a plan view of the model surgical snare of FIG. 4 showingpower loss density into blood;

FIG. 7 is a graph showing return loss (impedance match) into blood forthe model surgical snare of FIG. 4 with different tip diameters;

FIG. 8 is a plan view of the model surgical snare of FIG. 4 showingpower loss density into blood with minimal protrusion of the probe intothe area encircled by the retractable loop;

FIG. 9 is a graph showing return loss (impedance match) into blood forthe model surgical snare of FIG. 8;

FIG. 10 is a graph showing return loss (impedance match) into blood forthe model surgical snare of FIG. 8 with different loop diameters;

FIG. 11 shows four plan views of the model surgical snare of FIG. 8showing power loss density into blood for four different loop diameters;

FIGS. 12A, 12B and 12C show a schematic cross-sectional view of asurgical snare that is a fourth embodiment of the invention, in a polypcapture position, a deployed antenna position and a retracted positionrespectively;

FIG. 13A shows a schematic cross-sectional view of a distal portion of asurgical snare that is a fifth embodiment of the invention; and

FIG. 13B is a perspective view of a cap used in the surgical snare shownin FIG. 13A.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

FIG. 1A shows a cross-sectional view through a surgical snare 100 thatis an embodiment of the invention. The drawing is schematic and not toscale. In particular, the relative length of the device is shortenedsubstantially. In practice, the largest width (diameter) of the deviceis less than 2.6 mm in order to make it suitable for passing through theinstrument channel of an endoscope. The total length of the device,meanwhile, may be 2 m or more.

The surgical snare 100 comprises a coaxial cable 102, comprising aninner conductor 104, an outer conductor 106 and a dielectric material108 separating the inner conductor 104 from the outer conductor 106. Amicrowave connector 110 (e.g. a QMA connector or the like) is mounted ata proximal end of the coaxial cable 102 for connecting to a microwavesignal generator (not shown). A snare base 112 (e.g. a disc of asuitable insulator, e.g. a low loss microwave ceramic, PTFE, PEEK, Nylonor the like, is mounted at a distal end of the coaxial cable 102. The

The coaxial cable 102 is encased in a sleeve 114. The sleeve 114 has apair of passages for conveying a pair of pull wires 116 from theproximal end of the device to the distal end. Each pull wire 116 passesthrough the snare base 112 via a feed channel (i.e. a passageway formedin the snare base). The pair of pull wires 116 are each connected attheir distal end to a respective end 117 of a length of wire 118 thatforms a loop for the snare. The pair of pull wires 116 are eachconnected at their proximal end to a slider mechanism 120 which ismovable relative to the sleeve 114. The slider mechanism 120 can beoperated by the user to adjust the length of wire 118 that protrudesfrom the sleeve 114, thereby controlling the diameter of the loop formedby the length of wire 118 at the distal end of the device. The length ofwire 118 may have a shape retaining property which allows it to deformin order to enter the passages in the sleeve, but recover its loop shapewhen drawn out again. FIG. 1A shows the loop in a fully deployedposition. FIG. 1B shows the device with the loop partly withdrawn intothe sleeve 114.

In this embodiment, the inner conductor 104 of the coaxial cable 102protrudes through and beyond the snare base 112 to form an elongateconductive member 122. The function of the elongate conductive member122 is as a microwave antenna (preferably a radiating monopole antenna)to radiate microwave frequency energy supplied to it through the coaxialcable 102. The elongate conductive member 122 may or may not penetratethe biological tissue that is encircled by the loop of the snare (e.g.the stem of a polyp), depending on its length. The elongate conductivemember 122 includes a proximal portion that runs alongside the pullwires 116 in the snare base 112. Microwave energy delivered to theelongate conductive member 122 is coupled to set up a travelling wave inthe pull wires 116 at this location, from where it is conveyed into andradiates from the wire loop 118. The strength of the radiated field isat a maximum at the distal end of the loop, where the travelling wavesfrom each of the pull wires meet.

The microwave energy delivered to the elongate conductive member isradiated into the tissue, where it will promote coagulation andtherefore assist in the removal of the biological tissue or preventbleeding which would otherwise occur if mechanical action only wasemployed. It may be preferable to deliver microwave radiationcontinuously when a mechanical force is applied to the polyp stalk.Alternatively, the microwave source may be activated based on themeasurement of a physical force, e.g. measured using a mechanical toelectrical transducer, such as a piezoelectric transducer force sensoror the like.

The microwave energy may be delivered as a sequence of pulses or a burstof microwave energy, whereby the mechanical force follows or is embeddedwithin the burst of microwave coagulation energy. For example, oneactivation profile may comprise applying 10 W of microwave power for 10seconds, and applying the mechanical force for shorter periods withinthat 10 second time frame., i.e. the mechanical and microwave energy aredelivered together and microwave energy is always applied, butmechanical energy is applied at intervals within the window ofapplication of the microwave energy.

It may also be desirable to deliver the microwave energy based on thedetection of a change in the reflected signal caused by a change in theimpedance of the tissue that makes contact with the radiating monopole(or other) antenna, i.e. only deliver the microwave energy when theimpedance of blood is detected. In addition, the delivery of themicrowave energy may cease when a change of impedance is detected, i.e.the impedance of coagulated blood is detected. The measurementinformation may be magnitude only or magnitude and phase or phase only.To achieve this function effectively, the electrical length of theelongate conductive member 122 is determined based on a knowledge of thedielectric constant ε_(r) of the biological tissue to be treated, theequivalent dielectric properties of the structure surrounding theelongate conductive member 122 in the snare base 112, and the frequencyf of the microwave frequency energy that will be provided through thecoaxial cable. This information is used to calculate a wavelength λ_(L)of the microwave energy as it propagates through the biological tissue.The electrical length of the elongate conductive member 122 is set to bean odd number of quarter wavelengths, i.e.

$\frac{( {{2n} - 1} )\lambda_{L}}{4},{{{where}\mspace{14mu} \lambda_{L}} = \frac{c}{f\sqrt{ɛ_{r}}}}$

and c is the speed of light at the frequency of choice.

To avoid damaging the elongate conductive member 122 as the device isinserted along the instrument channel of an endoscope, a slidabletubular cover 124 is mounted at the distal end of the sleeve 114. A pullwire 126 extends from the tubular cover 124 to a handle 128 at theproximal end of the snare. The handle 128 may be operated by the user toslide the cover 124 over the elongate conductive member 122 (as shown inFIG. 1B). In use, the cover 124 is slid back over the sleeve 114 toexpose the elongate conductive member 122.

The wire loop 118 may be rotated by turning a handle 125 that isattached to the sleeve 114. The sleeve may include a braided cable whichfacilitates accurate torque transfer to allow the rotation of the wireloop to be controlled precisely.

FIG. 2A shows a cross-sectional view through a surgical snare 200 thatis another embodiment of the invention. Similarly to FIGS. 1A and 1B,the drawing is schematic and not to scale. Features in common with FIGS.1A and 1B are given the same reference numbers and are not describedagain. The handle 125 is omitted for clarity.

In FIG. 2A the inner conductor 104 of the coaxial cable 102 is connectedto a curved conductive portion 130 which comprises a pair of curvedprongs which extend symmetrically away from the feed point 132 at whichthey are connected to the inner conductor 104. Each prong may be aflexible elongate conductor, e.g. a wire or tube. In this embodiment,the length of wire 118 that forms a loop for the snare is fixed at oneend to a distal end 134 of one of the prongs. The other end of thelength of wire 118 is connected to the distal end 136 of a pull wire116. The proximal end of the pull wire 116 is connected to the slider120, which operates in the same manner as discussed above with referenceto FIGS. 1A and 1B.

However, in this embodiment, the pull wire 116 and length of wire 118forming the loop for the snare are arranged to pass through a guidepassage formed in one of the prongs. Thus, upon exiting the passage inthe sleeve 114, the pull wire 116 or wire 118 pass through a rearopening 138 on one of the prongs, through a hollow guide passage in thatprong, to exit through a front opening 140 at the distal end of thatprong.

The function of the curved conductive portion 130 is the same as theelongate conductive element 122 discussed above: it is a radiatingmicrowave monopole antenna for radiating microwave frequency energysupplied to it through the coaxial cable 102. In use, the curvedconductive portion 130 will contact the biological tissue that isencircled by the loop of the snare (e.g. the stem of a polyp). Themicrowave energy will therefore be radiated into the tissue, where itwill promote coagulation and therefore assist in the removal of thebiological tissue. To achieve this function effectively, the electricallength of the curved conductive portion 130 is therefore determined in asimilar way to the elongate conductive element 122 discussed above, i.e.it is determined based on a knowledge of the dielectric constant ε_(r)of the biological tissue to be treated and the frequency f of themicrowave frequency energy that will be provided through the coaxialcable. This information is used to calculate a wavelength λ_(L) of themicrowave energy as it propagates through the biological tissue. Theelectrical length of the curved conductive member 130 is thus set to bean odd number of quarter wavelengths, i.e.

$\frac{( {{2n} - 1} )\lambda_{L}}{4},{{{where}\mspace{14mu} \lambda_{L}} = \frac{c}{f\sqrt{ɛ_{r}}}}$

and c is the speed of light.

However, as the curved conductive portion 130 does not penetrate tissue,it can be made longer than the elongate conductive element 122. In orderto fit down the instrument channel of an endoscope, the prongs of thecurved conductive portion 130 preferably deform when the cover 124 isslid over them, as shown in FIG. 2B. The prongs may be resilientlydeformable so that they regain their original position when the cover124 is slid back over the sleeve 114.

FIG. 3A shows a cross-sectional view through a surgical snare 300 thatis another embodiment of the invention. Similarly to FIGS. 1A and 1B,the drawing is schematic and not to scale. Features in common with FIGS.1A and 1B are given the same reference numbers and are not describedagain.

In FIG. 3A the inner conductor 104 of the coaxial cable is connected toa conductive portion 142 which is mounted on the wire 118 that forms theloop for the snare. The wire 118 in this embodiment is made from anon-conductive material (e.g. nylon).

Similarly to the other embodiments discussed above, the function of theconductive portion 142 is the same as the elongate conductive element122 is as a radiating microwave monopole antenna for radiating microwavefrequency energy supplied to it through the coaxial cable 102. In use,the conductive portion 142 will contact the biological tissue that isencircled by the loop of the snare (e.g. the stem of a polyp). Themicrowave energy will therefore be radiated into the tissue, where itwill promote coagulation and therefore assist in the removal of thebiological tissue. To achieve this function effectively, the electricallength of the conductive portion 142 is therefore determined in asimilar way to the elongate conductive element 122 discussed above, i.e.it is determined based on a knowledge of the dielectric constant ε_(r)of the biological tissue to be treated and the frequency f of themicrowave frequency energy that will be provided through the coaxialcable. This information is used to calculate a wavelength λ_(L) of themicrowave energy as it propagates through the biological tissue. Theelectrical length of the conductive member 142 is thus set to be an oddnumber of quarter wavelengths, i.e.

$\frac{( {{2n} - 1} )\lambda_{L}}{4},{{{where}\mspace{14mu} \lambda_{L}} = \frac{c}{f\sqrt{ɛ_{r}}}}$

and c is the speed of light. It should also be noted that theconductivity and the dielectric constant of the biological tissue are afunction of the frequency of the microwave energy, and these parameters,together with the physical geometry of the antenna and the power lever(or energy delivery profile) determine the depth of penetration of theelectric field into the tissue structure, e.g. polyp stem, mucosa, etc.,which determines the profile of the focused heat.

Alternatively, however, the conductive member 142 may itself be acoaxial cable with an inner conductor electrically connected to theinner conductor 104 of the coaxial cable 102 and a ground outerconductor. The inner and outer conductors may be connected together atthe distal end 144 of the conductive portion 142, e.g. where itconnected to the wire 118. This structure may be made to radiate byremoving periodically spaced sections of the outer conductor. Thesections may be spaced by an odd number of quarter wavelengths, i.e.

$\frac{( {{2n} - 1} )\lambda_{L}}{4}.$

This structure is also known as a ‘leaky feed’.

In this embodiment, the length of wire 118 that forms a loop for thesnare is fixed at one end to a distal end 144 of the conductive portion132. The other end of the length of wire 118 is connected to the distalend 136 of a pull wire 116. The proximal end of the pull wire 116 isconnected to the slider 120, which operates in the same manner asdiscussed above with reference to FIGS. 1A and 1B.

The conductive element 142 may be deformable in a manner similar to thatshown in FIGS. 2A and 2B when the cover 124 is slid forward as shown inFIG. 3B. The conductive portion 142 or the wire 118 may be resilientlydeformable so that they regain their original position when the cover124 is slid back over the sleeve 114.

FIG. 4 depicts a representative model 400 of a surgical snare accordingto the invention that was modeled using CST MICROWAVE STUDIO®, and theperformance simulated as various modifications were made to thestructure to improve the return loss (impedance match into tissue loadmodel) and power density in the tissue.

In order to allow room for the mechanism to mechanically operate thesnare, the coaxial cable 402 required to feed microwave energy down theendoscope channel is selected to have a diameter that is around 1.2 mmin diameter. Sucoform 47 (made by Huber+Suhner) is a suitable cable thatis 1.2 mm in diameter and is flexible enough to allow full manipulationof the endoscope with the cable within its channel. Sucoform 86 cable,with an outside diameter of around 2.2 mm may also be a suitablecandidate for implementing the microwave snare.

The retractable loop 404 of the snare was modeled as a circular loop ofsquare cross section wire of thickness 0.5 mm. For most of thesimulations the internal diameter of the loop was 3.6 mm. This impliesthat the length of the antenna that will radiate into the stalk of thepolyp is around 11 mm. Referring to FIG. 11, the loop was filled with acylinder of tissue which for most of the simulations was given themicrowave properties of blood. The loop is connected to two wires 406which run beside the outer conductor of the coaxial cable 402, andoverlap it by one wire thickness. No further wire length was modeled.The inner conductor and dielectric covering 408 were extended from theend of the coaxial cable 402 to project into the loop, and the end ofthe center conductor was connected to a spherical metal dome 410.

The structure of FIG. 4 was the result of some preliminary modeling,during which it was found that the return loss could be improved bymoving the loop further from the end of the coaxial cable, and extendingthe inner conductor and dielectric covering 408.

The power density inside the loop is higher if the end of the centerconductor is exposed than if it is covered with dielectric. However, ifthe end of the center conductor is kept at its original radius the powerdensity close to its end is extremely high. Thus, placing a conductingdome on the end of the center conductor increases the power density inthe loop and results in less concentrated power close to the conductor.

FIG. 5 shows the return loss for the configuration shown in FIG. 4, witha long cylinder of blood completely filling the loop. The dielectricproperties of blood used in this simulation were as follows:

Pene- Conductivity Relative Loss Wavelength tration [S/m] permittivitytangent [m] depth [m] Blood 6.5057 52.539 0.38376 0.0070075 0.006019

FIG. 6 shows the power loss density in the plane of the loop. Here ithas been assumed that the specific heat capacity of blood is about 4.2J/(g·K), which is the specific heat capacity of water, and that thedensity of tissue is about 1 g/cm³, which is the density of water, sothat the volumetric heat capacity of tissue is about 4.2 J/(cm³·K).

Most of the area surrounding by the loop has a power absorption ofaround 67 dBW/m³, which is equivalent to 5 W/cm³, for a 1 W input power.Thus, for a 10 W input power the power absorption would be 50 W/cm³.This is enough to raise the temperature of the tissue in the loop by 12Ks⁻¹. Close to the spherical dome the temperature rise will beconsiderably faster.

FIG. 7 illustrates the effect on the return loss of changing thediameter of the spherical tip. Line 412 represents a diameter of 0.6 mm.Line 414 represents a diameter of 0.8 mm. Line 416 represents a diameterof 1.0 mm. Line 418 represents a diameter of 1.2 mm. Smaller tipdiameters give better return loss. However a larger diameter gives abetter heat distribution and minimizes the risk of perforation. Adiameter of 0.8 mm was chosen for further simulations.

FIG. 8 shows the results of a simulation carried out on a structurewhere there is minimal protrusion of the spherical tip into the loop. Inthis arrangement it is intended for only the metal sphere to protrudeinto tissue captured by the loop. FIG. 8 shows the power loss densityfor the structure. It is slightly lower than with the fully protrudingtip of FIG. 4. The central region of the loop in this arrangement has apower absorption level of around 64 dBW/m³. FIG. 9 shows the return lossfor the same structure.

FIGS. 10 and 11 illustrate the effect of changing the diameter of theloop using the configuration of FIG. 8. FIG. 10 shows the return lossfor six different diameters: 4 mm, 3.5 mm, 3 mm, 2 mm, 1.5 mm and 1 mm,represented by lines 420, 422, 424, 426, 428 and 430 respectively. At5.8 GHz the return loss for each diameter is as follows:

Loop diameter (mm) Return loss (dB) 1.0 −2.789259 1.5 −2.2937289 2.0−2.1571845 3.0 −2.4899045 3.5 −3.2297901 4.0 −3.8561229

As the loop diameter reduces, at first the return loss worsens, but fordiameters less than 2 mm the return loss begins to improve again (thehigher the magnitude of the return loss, the better the impedance matchinto tissue or the more power will be delivered into the tissue).

FIG. 11 shows the power densities in the cylinder of blood enclosed inthe loop for four loop diameters: 4 mm, 3 mm, 2.5 mm, 2 mm, and 1.5 mm.(The power density for a loop diameter of 3.6 mm is already shown inFIG. 8). These results show that the microwave power is adequate forloop diameters out to 4 mm and beyond. Given the stability of theprofile, there is tolerance of loop shape too, i.e. the loop may take alarge variety of shapes without disturbing the power absorption profile.For smaller diameters, even though the return loss get worse, the powerdensity rises, particularly in the center of the loop, which means thatthe microwave heating becomes stronger as the loop tightens. Thus, thepower density in the central region of the 4 mm loop is around 60dBW/m³, whereas in the central region of the 2 mm loop it is around 67dBW/m³.

The Sucoform 47 cable has an attenuation of about 3 dB/m at 5.8 GHz.This has an impact on the power that can be delivered to the end of thecable. The Sucoform 47 cable needs to be slightly longer than theendoscope channel, i.e. just over 2 m long, and so has an attenuation ofabout 7 dB. If the power available at the proximal end of the cable is50 W (47 dBm), the maximum power than can be delivered at the distal endof the cable is about 10 W (40 dBm).

FIG. 12A shows a cross-sectional view through a surgical snare 500 thatis another embodiment of the invention. Similarly to FIGS. 1A and 1B,the drawing is schematic and not to scale. Features in common with FIGS.1A and 1B are given the same reference numbers and are not describedagain.

This embodiment differs from the arrangement shown in FIGS. 1A and 1B inthat instead of having a sliding cover, the coaxial cable 102 isslidable within the sleeve 114 to cause the elongate conductive member122 to protrude into the area encircled by the retractable loop 118.This embodiment therefore comprises a housing 502 at the proximal end ofthe device. The housing 502 has a tapered distal tip 504 which isattached, e.g. adhered or otherwise secured, to the proximal end of thesleeve 114. The housing 502 has a passageway therethrough for receivingthe coaxial cable 102 in a manner that permits the coaxial cable 102 toslide relative to the housing 502 (and therefore the sleeve 114).

A handle 506 for operating the retractable loop 118 independently of theelongate conductive member 122 is slidably mounted on the housing 502and connected to a proximal end of a push rod 508. The push rod 508extends through the sleeve 114 and is attached at its distal end to afirst end of the retractable loop 118.

This embodiment comprises a snare base 512 that is fixed, e.g. adheredor otherwise secured, to the distal end of the sleeve 114. As shown inthe expanded cross-sectional view of FIG. 12B, the snare base 512 hastwo longitudinal passageways therethrough. A first passageway 514 is forconveying the push rod 508. The distal end 117 of the push rod 508 thatis connected to the first end of the retractable loop 118 is locatedwithin the first passageway 514 in this view. A second passageway 516 isfor conveying the coaxial cable 102. The snare base 512 also receivesthe second end 518 of the retractable loop 118. The second end 518 isattached to the snare base 512.

FIG. 12A shows the surgical snare 500 of this embodiment in aconfiguration where the elongate conductive member is retracted but theretractable loop 118 is extended. This may correspond to a polyp captureposition, where the retractable loop is open to fit over a polyp.

FIG. 12B shows the surgical snare 500 of this embodiment in aconfiguration where the elongate conductive member 122 is extended intothe area encircled by the retractable loop 118. This may correspond to adeployed antenna position in which the elongate conductive member 122may deliver microwave frequency energy into tissue captured within theretractable loop 118. To arrive in this configuration from the polypcapture configuration shown in FIG. 12A, the coaxial cable 102 is moveddistally (to the right as shown in FIG. 12B by arrow 522). In thisembodiment, the elongate conductive member 122 has a rounded conductivetip 520 mounted thereon. The rounded conductive tip 520 may be formedfrom silver wire wrapped around and soldered to elongate conductivemember 122, i.e. to the protruding portion of the inner conductor 104.

FIG. 12C shows the surgical snare 500 of this embodiment in aconfiguration where both the retractable loop 118 and the elongateconductive member 122 are fully retracted. This may correspond to aretracted position, e.g. for use when moving the device through theinstrument channel of an endoscope. To arrive in this configuration fromthe polyp capture configuration shown in FIG. 12A, the handle 506 ismoved proximally (to the left as shown in FIG. 12C by arrow 524).

The process of retraction may be used to assist cutting of biologicaltissue (e.g. a polyp stem) encircled by the retractable loop 118. Theretractable loop may force the encircled tissue against the distalsurface of the snare base 512, which thus forces a reaction surface toassist cutting. the distal surface of the snare base may be shaped toassist cutting, e.g. by being curved in a convex manner. The retractableloop 118 may have a roughened or sharpened surface (e.g. on the insidethereof) to assist cutting.

FIG. 13A shows a schematic cross-sectional view of a distal portion of asurgical snare 600 that is another embodiment of the invention. Thisembodiment may use the same deployment mechanism (housing 502 and handle506) as FIG. 12A, and so these feature are omitted for clarity. Featuresin common with FIGS. 1A and 1B and FIGS. 12A, 12B and 12C are given thesame reference numbers and are not described again. Similarly to FIGS.1A and 1B, the drawing is schematic and not to scale.

Similarly to the embodiment discussed with reference to FIGS. 12A, 12Band 12C above, in the embodiment of FIG. 13A the coaxial cable 102 isslidable with the sleeve 114 in order to extend and retract the elongateconductive member 122. Similarly, the retractable loop 118 is operatedvia the slidable push rod 508 in the same way as discussed above.

However, the configuration of the snare base in FIG. 13A is differentfrom FIGS. 12A, 12B and 12C. In this embodiment, the snare basecomprises a cap 602 that is secured to the end of the sleeve 114. Asshown in FIG. 13B, the cap 602 has a top hat shape, with an annularflange 604 that provides the distal surface thereof which mounted inuse. The annular flange 604 may thus provide the reaction surface duringmechanical cutting using the retractable loop 118. The cap has apassageway 606 therethrough for conveying the coaxial cable 102 and thepush rod 508 or retractable loop 118.

Within the sleeve 114, a collar 608 is attached (e.g. adhered orsoldered or otherwise affixed) to the outer surface (e.g. outerconductor 106) of the coaxial cable 102. The collar 608 thus moves withthe coaxial cable 102 within the sleeve 114. The collar 608 has a largerdiameter than the coaxial cable 102 and therefore leaves a space betweenits inner surface and the outer surface of the coaxial cable on a sideof the coaxial cable that is opposite to the location at which thecollar is attached to the coaxial cable. The push rod 508 passes throughthis space and is thus free to move relative to the coaxial cable 102.

The inner diameter of the flange 604 is smaller than the diameter of thecollar 608 to act as a stop to limit the extent to which the elongateconductive member 122 can protrude out of the sleeve 114.

In this embodiment the other end 518 of the retractable loop 518 isattached (e.g. soldered) to the collar 608, e.g. to the outer surface ofthe collar 608. This means that the attachment point of the retractableloop 118 lies inside the sleeve 114, which may assist in completeretraction of the loop. Moreover, since the collar 608 is movable withthe coaxial cable 102 within the sleeve 114, both ends of theretractable loop 118 are effectively movable within the sleeve, whichcan ensure that the loop is fully retractable.

1. A surgical snare comprising: a retractable loop of conductivematerial for encircling an area containing biological tissue; aradiating structure arranged to radiate microwave frequency energy intothe area encircled by the retractable loop; and a coaxial cable forconveying microwave frequency energy to the radiating structure, thecoaxial cable comprising an inner conductor, an outer conductorsurrounding and coaxial with the inner conductor, and a dielectricmaterial separating the inner conductor and the outer conductor, whereinthe radiating structure comprises: an elongate conductive memberconnected to the inner conductor of the coaxial cable and beingelectrically insulated from the outer conductor of the coaxial cable,and a snare base at a distal end for the coaxial cable, the snare basehaving a feed channel for conveying a length of the conductive materialthat forms the retractable loop, wherein the elongate conductive membercomprises a distal portion that protrudes into the area encircled by theretractable loop to act as a radiating microwave monopole antenna, and aproximal portion that extends through the snare base alongside the feedchannel.
 2. A surgical snare according to claim 1, wherein theelectrical length of the elongate conductive member may be about$\frac{( {{2n} - 1} )\lambda_{L}}{4},$ where λ_(L) is thewavelength of the microwave frequency energy along the proximal portionof the elongate conductive element, i.e. the wavelength in the snarebase, and n is a positive integer.
 3. A surgical snare according toclaim 1, wherein the distal portion of the elongate conductive member isshaped to penetrate biological tissue.
 4. A surgical snare according toclaim 1, wherein the elongate conductive member is coated in abiocompatible material.
 5. A surgical snare according to claim 1,wherein the snare base comprises an insulating disc.
 6. A surgical snareaccording to claim 1, wherein the coaxial cable is encased in a sleevesuitable for insertion through the instrument channel of an endoscope.7. A surgical snare according to claim 6, wherein the sleeve comprises arotatable braided cable to permit adjustment of an orientation of theplane of the retractable loop.
 8. A surgical snare according to claim 1,wherein the retractable loop comprises a wire that extends beyond thedistal end of the coaxial cable, the wire being arranged to naturallyadopt a looped shape between two ends located at the distal end of thecoaxial cable.
 9. A surgical snare according to claim 8, wherein theretractable loop is adjustable to vary the length of wire between thetwo ends.
 10. A surgical snare according to claim 1, wherein theelongate conductive member is retractable independently of theretractable loop.
 11. A surgical snare according to claim 10 including acover mounted at the distal end of the coaxial cable, and slidablebetween a covering position in which it overlaps the elongate conductivemember and a retracted position in which the elongate conductive memberprotrudes therefrom.
 12. A surgical snare according to claim 1, whereinmicrowave frequency is in the range 1 GHz to 60 GHz.
 13. Electrosurgicalapparatus comprising: a microwave signal generator for outputtingmicrowave frequency energy having a frequency of 1 GHz or more, and asurgical snare according to claim 1 connected to receive the microwavefrequency energy and deliver it through the coaxial cable to be emittedas a microwave frequency field by the elongate conductive member.
 14. Asurgical snare comprising: a retractable loop for encircling an areacontaining biological tissue; a radiating structure arranged to radiatemicrowave frequency energy into the area encircled by the retractableloop; and a coaxial cable for conveying microwave frequency energy tothe radiating structure, the coaxial cable comprising an innerconductor, an outer conductor surrounding and coaxial with the innerconductor, and a dielectric material separating the inner conductor andthe outer conductor, wherein the radiating structure comprises a curvedconductive portion partially bounding the area encircled by theretractable loop, the curved conductive portion being connected to theinner conductor of the coaxial cable and electrically insulated from theouter conductor of the coaxial cable to act as a radiating microwavemonopole antenna.
 15. A surgical snare according to claim 14, whereinthe curved conductive portion extends between two ends, which are spacedat equal distances from a connection point at which the inner conductorof the coaxial cable is connected to the curved conductive portion. 16.A surgical snare according to claim 15, wherein the electrical lengthbetween the ends of the curved conductive portion is$\frac{( {{2n} - 1} )\lambda_{L}}{4},$ where λ_(L) is thewavelength of the microwave frequency energy when propagating throughthe biological tissue, and n is a positive integer.
 17. A surgical snareaccording to claim 15, wherein the electrical length of the curvedconductive portion is 10 mm or more.
 18. A surgical snare according toclaim 14, wherein the curved conductive portion comprises a pair offlexible prongs extending from the distal end of the coaxial cable. 19.A surgical snare according to claim 18 having a tubular end cap mountedat the distal end of the coaxial cable, wherein the curved conductiveportion and tubular end cap are movable relative to each other between:a stored configuration in which the curved conductive portion issurrounded by the tubular end cap, and a deployed configuration in whichthe curved conductive portion protrudes beyond a distal end of thetubular end cap.
 20. A surgical snare according to claim 19, wherein thetubular end cap has an outer diameter less than 2.6 mm.
 21. A surgicalsnare according to claim 14, wherein the curved conductive portioncomprises a guide for the retractable loop.
 22. A surgical snareaccording to claim 21, wherein the guide comprises a hollow tubularsection with an opening at one end thereof, wherein the retractable loopextends along the hollow tubular section and through the opening.
 23. Asurgical snare according to claim 22, wherein the opening of the hollowtubular section is at a first end of the curved conductive portion andwherein the retractable loop includes a wire that is fixed to a secondend of the curved conductive portion.
 24. Electrosurgical apparatuscomprising: a microwave signal generator for outputting microwavefrequency energy having a frequency of 1 GHz or more, and a surgicalsnare according to claim 14 connected to receive the microwave frequencyenergy and deliver it through the coaxial cable to be emitted as amicrowave frequency field by the curved conductive portion.
 25. Asurgical snare comprising: a retractable loop for encircling an areacontaining biological tissue; a radiating structure arranged to radiatemicrowave frequency energy into the area encircled by the retractableloop; and a coaxial cable for conveying microwave frequency energy tothe radiating structure, the coaxial cable comprising an innerconductor, an outer conductor surrounding and coaxial with the innerconductor, and a dielectric material separating the inner conductor andthe outer conductor, wherein the radiating structure comprises aconductive portion formed in or on the retractable loop, the conductiveportion being connected to receive microwave power from the coaxialcable and configured to radiate the received microwave frequency energyinto the area encircled by the retractable loop.
 26. A surgical snareaccording to claim 25, wherein the electrical length of the conductiveportion around the retractable loop is$\frac{( {{2n} - 1} )\lambda_{L}}{4},$ where λ_(L) is thewavelength of the microwave frequency energy when propagating throughthe biological tissue, and n is a positive integer.
 27. A surgical snareaccording to claim 25, wherein the retractable loop comprises a wirehaving a first end fixed at the distal end of the coaxial cable, and asecond end whose position relative to the distal end of the coaxialcable is adjustable, and wherein the conductive portion extends aroundthe retractable loop from the first end.
 28. A surgical snare accordingto claim 25, wherein the conductive portion comprises a length ofcoaxial cable that is shorted at its distal end and along which portionsof the outer conductor are periodically removed to permit radiationtherefrom.
 29. Electrosurgical apparatus comprising: a microwave signalgenerator for outputting microwave frequency energy having a frequencyof 1 GHz or more, and a surgical snare according to claim 25 connectedto receive the microwave frequency energy and deliver it through thecoaxial cable to be emitting as a microwave frequency field by theconductive portion.